DE2731220A1 - AFTER-TREATMENT FOR CHEMICAL COATINGS ON ZINC OR ZINC ALLOYS - Google Patents

Description

The invention relates to an improved method of aftertreatment of chemically produced coatings on zinc or zinc alloys to improve the adhesive strength and the corrosion resistance of organic coatings.

It is common to use zinc or zinc alloys to improve their corrosion resistance and increase them to apply phosphate or oxide layers to the paint adhesion. The properties of the coating can still continue can be improved if, for example, the phosphate layer is sealed will.

It is known that the coating of zinc or its alloys with chromate increases the corrosion resistance increased, but the paint adhesion and scratch resistance is not significantly improved. Significantly better results are achieved when the metal surface is provided with a phosphate or a complex oxide coating and then is sealed. For this purpose, the coated surface is treated with a solution containing chromic acid, dichromate or may contain chromates. Sealing the phosphate coating can also prevent the Paint adhesion decreases due to aging of the phosphate layer.

This sealing of the surface by rinsing with a chromium-containing solution is not expensive and is particularly effective very good corrosion resistance.

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However, the use of chromic acid or its salts presents great difficulties in handling and processing of the resulting wastewater.

Various solutions are known for the aftertreatment of coated zinc surfaces, which instead of chromium-containing Solutions are used.

In addition to deroi-aminophosphoric acid or ol- amino sulfo acid (Japanese application 78531/1975) and free or complex-bound fluoride (USP 3,895,970), tannin (Japanese application 2902/1976) is also used in solutions for aftertreatment. Successes can be achieved with these processes, but the results are not satisfactory in comparison with aftertreatment with a chromium-containing solution, since the corrosion protection properties are poor and the formation of bubbles after painting is possible.

The invention is based on the object of developing a method which can be used for the aftertreatment of coated Surfaces made of zinc or its alloys is suitable, does not contain chromates and thus the difficulties of Wastewater treatment significantly reduced.

The object is achieved according to the invention in that a chromium-free aqueous solution is used which tannin and Contains at least 0.1 g of thiourea per liter of solution. As thiourea, thiourea itself or its Derivatives such as dimethyl thiourea, diethyl thiourea or guanyl thiourea. They should be in the treatment solution at a concentration of 0.1 to 20 g / l, preferably 0.5 to 5 g / l of solution.

If the concentration is below 0.1 g / l thiourea

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no improvement noted, while the use of more than 20 g / l can be disadvantageous in use and also becomes too expensive.

Hydrolyzable, condensed or partially hydrolyzed tannins or natural tannins such as gallotannin, Turkish Tannin, witch hazel tannin, sumac tannin, quebracho tannin, Catechin tannin, etc. The use concentration is expediently 0.1 to 20 g / l, preferably 0.5 to 3 g / l. The quantitative ratio of thiourea or its derivative the tannin can vary between 10: 1 to 1:10, preferably the ratio should be 3: 1 to 1: 3. When the amount ratio is substantially in the range of 10: 1 to 1: 10 deviates, after painting the surface there may be blistering in the paint surface and a reduced Corrosion protection come.

The most favorable range of the pH value of the treatment solution depends on the type of tannin used and the Application conditions and is normally between pH 2 and pH 10, preferably between pH 2.5 and pH 6.5

If the pH is above 10, the tannin may break down take place, while at a pH value below 2 the existing phosphate layer can be dissolved.

Various acidic or alkaline water-soluble substances can be used to regulate the pH. Since the residues of such substances on the metal surface can lead to the formation of bubbles in the paint layer, must these chemicals are carefully selected.

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The adjustment of the desired pH value can be done with any compatible acids or bases, which are usually used for this purpose. Usually only very small amounts are needed.

The aftertreatment solution can be applied to the coated surface made of zinc or a zinc alloy by dipping, spraying, brushing, rolling, etc. During use, the temperature of the aftertreatment solution is between room temperature and 90 ^° C., preferably 65 ^° C.

Usually the metal surface is cleaned immediately after the treatment dried. If there is an excess of soluble salts on the surface, add water if necessary getting washed.

The method according to the invention can result in paint adhesion and corrosion resistance on coated surfaces Zinc or zinc alloys can be achieved, which previously only achieved by post-treatment with chromium-containing solutions became.

Example;

Test pieces with dimensions of 100 300 χ 0.3 mm, hot-dip galvanized and treated with chromic acid, were polished 5 times by wet lapping to remove the adhered chromate from the surface to remove.

The test pieces were then placed in an aqueous solution of Parcolene Z (trade name of a pre-rinse by Nihon Parkerizing Co.) immersed in a concentration of 3 kg / l at room temperature for 3 seconds.

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Immediately thereafter, the parts between the rubber rolls to be freed from the excess solution and (Parkerizing Co. Bonderite no. 3305, commercial product of Nihon) sprayed with a commercially available aqueous phosphating solution in a concentration of 60 g / l at 65 ⁰ C for loose customer, rinsed with water and freed from the excess liquid between rubber rollers.

The test parts pretreated and coated in this way are then post-treated differently.

Sample 1 (according to the invention) immersed in an aqueous solution of

2 g / l thiourea and 1 g / l gallotannin for 2 seconds at 50 ^° C.

Comparative sample ^ J

Immerse in an aqueous solution of 2 g / l thiourea.

Comparison sample_ 2

Immerse in an aqueous solution of 2 g / l gallotannin.

Comparison sample 3

Immersion in an aqueous chromic acid solution containing 18 g / l Parcolene No. 62 (commercial product of Nihon Parkerizing Co.).

Comparison sample 4

No post-treatment of the phosphated test panels.

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After drying, some of the sample sheets were coated with a base coat based on alkyd resins.

After baking the lacquer in 50 seconds at 280 ^° C. in an air circulation oven, a coating with a thickness of 6 microns was obtained.

The other part of the sample was coated after drying with a primer based on epoxy resins, which, after baking in 50 seconds in a forced air oven at 280 ⁰ C gave a coating of 4 microns.

The thus treated panels were then provided with a top coat based on acrylic resins which gives a double coating with a total thickness of 14 microns after baking in 60 seconds at 280 ⁰ C in a forced air oven.

To assess the effect of the aftertreatment solution, the test parts were tested as follows:

The paint adhesion was tested in a bending test, whereby the metal sheets were bent by 180 °, then folded sharply and at the bending point have been provided with a cellophane tape, which is then quickly peeled off again.

The amount of paint removed is then determined and rated from 5 to 1.

For the corrosion test, the test parts are scratched crosswise with a knife down to the metal and the salt spray test exposed.

The test time for the parts with the base coat is 240 hours and for the parts with the double coat 1000 hours.

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After the salt spray test, the test parts are washed, dried and provided with a cellophane tape on the scratched surface, which is then quickly put back is deducted.

The maximum width (in mm) of the lacquer layer removed on both sides of the scratched area was measured the removal of the paint layer due to the formation of bubbles.

Another corrosion test consists in that the parts are immersed in boiling water for 2 hours and then be subjected to the bending test.

The results obtained are summarized in the table below.

- Tabel -

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Panels with base coat Aftertreatment
solution Salt spray test 240 hours Corrosion
width (mm Bend
test Sheet metal with double coating Bend
test Salζ spray test
1000 hours urea
2 g / l
Gallotannin
1 g / l Bubbles in
Paint film O - 0.5 4th Dipping into the boiling
Water 2, hours 4th Corrosion
width (mm Pattern 1 urea
2 g / l no 3 2 Bubbles in
Paint film 2 0 - 1 Comp.-
^ J pattern 1
<- » Gallotannin
2 g / l in places
small EQasai 3 2 no 2 4th u>
co comp, -
»Pattern 2 Chromic acid
solution in places
little Blasai O - 0.5 3-4 in places
little Blasai 4th 3 ο comp.
^ ³ pattern 3
OD untreated
I. no 5 2 in places
small bubbles 2 0 - 1 CO
Comp.-
Pattern 4 small bubbles
on the
whole bottle. no 5 little HLasai
a.the whole
area

Assessment of the bending test:

5: no paint removal

4: 5% paint removal

3: 25 % "

2: 50% "

1: more than 50 % paint removal

K)

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The table clearly shows that the use of the rinsing solution according to the invention results in paint adhesion and corrosion resistance is obtained which is comparable to the results obtained with use a chromic acid solution.

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Claims (3)

Claims 1. A process for the aftertreatment of chemically produced - ^ coatings on zinc or zinc alloys, characterized in that a chromium-free aqueous solution is used which contains tannin and at least 0.1 g of thiourea per liter of solution. 2. The method according to claim 1, characterized in that an aqueous solution with tannin and 0.1 to 20, preferably 0.5 to 5 g of thiourea per liter of solution is used. 3. The method according to claim 1 and 2, characterized in that the weight ratio between thiourea and tannin in the aqueous solution used is 10: 1 to 1:10 and preferably 3: 1 to 1: 3. 709884/0783 ORIGINAL INSPECTED